This invention relates generally to systems utilizing a cathode ray tube (CRT) and more particularly to a means for preventing burn-in effects on the CRT caused by prolonged display of the same image.
To display an image on the face of a CRT, an electron beam is scanned across a phosphorescent coating on the inside of that face in successive horizontal lines which progress from top to bottom. The electron beam intensity is modulated with the information to be displayed so that the phosphorescent coating is caused to glow in a pattern corresponding to that image. Alternatively, the electron beam may be scanned by voltages representing successive pairs of vectors along the resultant of which the beam is to be moved. Normally, the image moves, as in television, so that no particular area of the screen is singled out for extended electron beam bombardment. Thus, the electron beam normally does not land on any given spot for a long enough time to damage the phosphorescent layer. The same is not true, however, when the displayed image is stationary and particularly when it is both stationary and sharply outlined. In such cases, the electron beam is traced repeatedly along the same path across the phosphorescent coated faceplate. The damage that results is what is called "burn-in" and it can leave a permanent and highly undesirable "scar" along which the phosphorescent coating is damaged. Consequently, when another image is subsequently sought to be displayed, the outline of the burn-in path may become partially or totally visible.
It is therefore a principal object of the present invention to prevent burn-in of a CRT by the sharply focused electron beams encountered in many applications, including but not limited to alphanumeric, line and spot displays of computer information; stationary boundaries of television games, highlights of stationary television scenes, especially those that occur with closed circuit fixed camera surveillance; and stationary images of ground based radar displays.
A more specific object of this invention is to provide optimized CRT burn-in protection through the use of a constant velocity, uniform coverage periodic electron beam traverse, which is comparatively simple in its structure, reliable in operation, and low in cost.
These and other objects of the invention are attained through the realization that the most suitable path for the movement of an electron beam image is one in which, over a prolonged time, a given displayed spot traverses uniformly all points contained within the extremes of its travel and traverses those extremes during each offset period. Such a path yields the highest probability that maximum intensity electron bombardment of any particular light emitting phosphor molecule will be of short duration compared to an offset period and that any such molecule will receive low average bombardment over one offset period; thus yielding the optimum protection against burn-in.
Such a uniformly varying path may be achieved by the use, in each axis deflection amplifier, of a triangular waveform offset voltage whose phase relationship with the triangular waveform offset voltage of the other axis varies uniformly. Other waveforms, such as the sine wave, could be used, but only the triangular waveform has the characteristic of uniform amplitude rate-of-change versus time. Any of several means can be used to generate the required waveforms, including motor driven potentiometers, mask movement between photocells and light sources, and constant current charging and discharging of capacitors. The latter means, utilizing transistor integrated circuits, offers many advantages, including the lack of moving parts, small size and weight, low cost, stability and indefinitely long life.